Known gas discharge lamps consist of a vessel containing a filling gas wherein the gas discharge takes place, and usually two metallic electrodes which are sealed in the discharge vessel. An electrode supplies the electrons for the discharge, which electrons are subsequently applied to the external current circuit via the second electrode. The donation of the electrons generally takes place via thermionic emission (hot electrodes), although it may alternatively be brought about by an emission in a strong electric field or, directly, via ion bombardment (ion-induced secondary emission) (cold electrodes). In an inductive mode of operation the charge carriers are generated directly in the gas volume by means of an electromagnetic alternating field of high frequency (typically higher than 1 MHz in the case of low-pressure gas discharge lamps). The electrons travel along closed paths inside the discharge vessel; customary electrodes are absent in this mode of operation. In a capacitive mode of operation, capacitive coupling-in structures are used as electrodes. These electrodes are usually embodied so as to be insulators (dielectric materials) which, on one side, are in contact with the gas discharge and, on the other side, are electroconductively connected (for example, by means of a metallic contact) to an external current circuit. When an alternating voltage is applied to the capacitive electrodes, an electric alternating field is formed in the discharge vessel and the charge carriers move on the linear electric fields of said alternating field. In the high-frequency range (f>10 MHz) the capacitive lamps are similar to the inductive lamps, because in this range the charge carriers are also generated in the entire gas volume. The surface properties of the dielectric electrode are in this case less important (so-called α discharge mode). At lower frequencies the mode of operation of the capacitive lamps changes and the electrons which are important for the discharge must be originally emitted at the surface of the dielectric electrode and multiplied in a so-called cathode drop region so as to maintain the discharge. Consequently, the emission behavior of the dielectric material then determines the functioning of the lamp (so-called γ discharge mode). The power deposited in the cathode drop region is not available to the generation of light and, consequently, reduces the efficiency of the lamp (lumen per Watt).
For many devices it is advantageous to use fluorescent lamps of small diameter (less than 5 mm) and an as high as possible luminous flux per unit of length of the lamp (lumen per cm). Moreover, most fields of application require a high resistance against switching transients for the lamp. This holds notably for the use of gas discharge lamps for backlighting for a liquid crystal display (LCD backlight).
Hot cathode lamps require a minimum diameter of the discharge vessel of approximately 10 mm in order to enable the coil and the anode shield to be accommodated. When the anode shield is dispensed with, inner diameters of approximately 6 mm can be realized, be it that the service life is strongly reduced due to the increased blackening. Moreover, the switching behavior of hot cathode lamps is unacceptable for many fields of application and, moreover, they can be dimmed only with difficulty.
Fluorescent gas discharge lamps having a small lamp diameter (no more than 5 mm) can thus far be realized only in the form of cold cathode lamps or in the form of capacitive gas discharge lamps with an operating frequency in the high-frequency range (higher than 1 MHz). Cold cathode lamps offer the advantage that they can be operated at low frequencies (30-50 kHz). Therefore, their electromagnetic radiation is only weak. However, the discharge current in cold cathode lamps is severely restricted (to a maximum value of approximately 10 mA). The current limitation is due to the strongly increased sputter rate of electrode material as a function of the discharge current. Moreover, the current limitation serves to prevent local heating of the electrode to such an extent that thermal emission occurs with a severely increased sputter rate. The released electrode material is then deposited in the discharge vessel and hence causes fast blackening of the lamp.
In the case of a capacitive discharge lamp with an operating frequency f>1 MHz, the high operating frequency causes, in conjunction with a high current density in the lamp (large current, small lamp diameter), strong electromagnetic radiation. This makes it necessary to take elaborate steps throughout the system formed by the lamp, reflector, drive electronics etc. in order to limit this electromagnetic radiation. Because the power is capacitively coupled in via the discharge vessel, the operating frequency is limited downwards (to approximately 1 MHz) via the capacitance of the coupling-in surface.
U.S. Pat. No. 2,624,858 discloses a capacitive gas discharge lamp provided with a dielectric layer between external electrodes and the gas discharge. The external electrodes are connected to an alternating current source which outputs a voltage of from 500 V to 10,000 V at a frequency of 120 Hz. The dielectric layer has a high dielectric constant ε which is greater than 100, preferably greater than 2000. The capacitive coupling in of the external alternating voltage by means of the dielectric layer causes ionization and excitation of the gas in the lamp, so that the luminous gas discharge occurs. This combination of dielectric constant and operating frequency is capable of achieving a high luminous flux of the lamp only by using coupling-in structures of very large dimensions so that the lamp overall will also be of large dimensions. Moreover, in such a lamp a high luminous flux requires an extremely high operating voltage and hence an expensive drive circuit. In addition, in this frequency range the secondary emission coefficient γ is significantly less attractive, so that the efficiency of the gas discharge is less and a smaller amount of light is generated.